Null Reference Exception Opening Project (#1681)

I have a project file that Software Ideas Modeler can't open anymore.

"An error occured while loading the project file."

System.NullReferenceException: Object reference not set to an instance of an object.

at SoftwareIdeasModeler.Core.Projecting.ConcreteDiagramElementBase.<>c.<get_AttachedParts>b__19_0(ConcreteDiagramElementBase x)

at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()

at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)

at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)

at SoftwareIdeasModeler.Core.Projecting.ConcreteDiagramElementBase.get_AttachedParts()

at SoftwareIdeasModeler.Core.Projecting.Project.BindOtherConcreteItems(Folder folder)

at SoftwareIdeasModeler.Core.Projecting.Project.BindEntities()

at SoftwareIdeasModeler.ProjectXml.SimXmlReader.Load()

at SoftwareIdeasModeler.ProjectHelper.LoadProject(String localFileName, Boolean showReadOnlyWarning, Boolean checkFileVersion, Boolean processCustomDiagrams, Boolean showErrorMessages)

at SoftwareIdeasModeler.AppCore.OpenProjectFile(String fileName, Form splash)